Softshell Clam Culture: Basic Biology and General Culture Considerations Scott Weston and Joseph K
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NRAC Publication No. 201-2010 University of Maryland, 2113 Animal Science Building College Park, Maryland 20742-2317 Telephone: 301-405-6085, FAX: 301-314-9412 E-mail: [email protected] Web: http://www.nrac.umd.edu Softshell Clam Culture: Basic Biology and General Culture Considerations Scott Weston and Joseph K. Buttner, Northeastern Massachusetts Aquaculture Center and Department of Biology, Salem State University, Salem MA Brian F. Beal, University of Maine at Machias and Downeast Institute for Applied Marine Research and Education, Beals, ME The softshell clam, Mya arenaria, (Figure 1) is an siphon extends through the sediments to the sedi- ecologically important bivalve mollusk that is harvested ment/water interface. Cilia associated with the gills recreationally and commercially from coastal waters of move oxygenated water with microscopic food, such as the western North Atlantic. As with all bivalve mollusks, phytoplankton, bacteria and small organic particles, they possess two valves or shells that are connected by a hinge. The pair of whitish/gray shells surrounds and pro- tects a mantle, visceral mass and muscular foot. The elongate shells are relatively thin and brittle, leading to the name “softshell.” Unlike hard clams and oysters, the softshell clam cannot close its shells completely, result- ing in a gape most notable at the anterior and posterior ends. As such, softshell clams are damaged more readi- ly and can have a shorter shelf-life than do similar-size oysters or hard clams. A shelf-life of 7 to 10 days can be expected when the animal is kept cool, approximately 3 to 4oC, and moist. The softshell clam may exceed 10 cm in shell length (SL) and lives burrowed in sediments. The clam moves by using its relatively small, muscular foot. Clams less than 25 mm SL use the muscular foot to burrow and to move laterally on the sediment surface; larger clams, which tend to be sedentary, use the muscular foot to move vertically in the sediments. Softshell clams pos- sess a single long, fleshy siphon composed of a fused, Figure 1. Softshell clams are typically white or gray and larger incurrent and a smaller excurrent tube. The fused cannot close completely. (Photo: Joe Buttner) through the incurrent tube into the visceral mass. The gills Life Cycle sieve out or filter food and other particles, and send it to the labial palps. The palps sort food items, directing some Softshell clams in nature routinely live for 10 to 12 to the mouth for ingestion. Other food items and non-food years and have been documented to live for 28 years. items are bundled in mucus expelled through the excur- Sexes are typically separate, but a small percentage of a rent tube as pseudofeces. Metabolic wastes and feces are population may be hermaphroditic. Male and female also expelled via the excurrent tube. Large clams may fil- clams are identical externally. The only non-intrusive ter 50 L of water per day depending on seawater temper- way to determine the sex of an individual is to observe atures. When disturbed, the siphon can be retracted its gametes released at spawning. Clams can develop quickly and a squirt of water is emitted. gametes at ~20 mm SL; by 30 mm SL most are able to reproduce. As water warms in the spring, gametes start Distribution to ripen or mature. This process can take 8 or more weeks. Spawning generally occurs after inshore water temperature reaches at least 10ºC. A second spawn may The softshell clam naturally ranges in coastal waters occur in the late summer or early fall in more southerly of the western North Atlantic from Labrador and the waters of the western North Atlantic, as water tempera- Canadian Maritimes south along the Atlantic shore to ture drops below 25ºC. One female can produce and North Carolina, but is not common in waters south of release in excess of one million eggs per spawn. Eggs Maryland. Outside its natural range, the clam is often con- are round and typically 40 to 50 µm (0.04 to 0.05 mm) sidered a nuisance species. Softshell clams were first in diameter. Sperm are 2 to 3 µm in length. observed on the U.S. Pacific coast in San Francisco in 1874. They have subsequently spread northward into Ore- Fertilization occurs externally as eggs and sperm are gon, Washington, British Columbia and Alaska waters. In released into the water column and encounter each other. Europe, this species has become established from Nor- After fertilization, the eggs pass through three plankton- way to Portugal and has been introduced into the Mediter- ic larval stages: the short-duration trochophore, the ranean Sea with reports of its existence along the northern veliger and finally the pediveliger (Figure 2). Tro- Adriatic, western Sicily, and the Saronikos Gulf in chophore larvae are characterized by the presence of rel- Greece. atively large apical cilia. A mouth, gut, and anus are present at this developmental stage, but no valves or shells occur. Veliger larvae are characterized by the pres- Habitat ence of a velum and very thin valves. The ciliated velum enables larval clams to swim and capture food. The pedi- To protect their thin and brittle shells, softshell clams veliger is the last larval stage before the animal settles to are normally found buried in a wide variety of sediment the bottom. It can be quite variable in duration as the types in protected bays and estuaries. The clam grows best young clam may metamorphose quickly or remain in substrates of mixed sand and silt. It will colonize and planktonic for several weeks depending on various envi- can attain high densities in clay, mud, silt and gravel sed- ronmental and biological cues. Temperature, salinity and iments. Clams in coarse sediments, such as gravel, typi- food availability affect duration and success of the larval cally have thicker and more deformed shells compared to stage. Typically, the larval stage is 8 to 21 days in dura- clams in finer sediments. Densities can exceed 100 mar- tion. The stage ends as planktonic larvae metamorphose 2 ket-size individuals (≥ 51 mm SL)/m . into benthic, juvenile clams, which lack a velum and possess a single-chambered siphon as well as a function- The eurythermal and euryhaline softshell clam al muscular foot as the sole means of locomotion. tolerates temperatures from -2 to 28ºC and salinities Recently metamorphosed benthic clams are small with a between 4 to 33 ppt. They prefer cooler temperatures, shell length of ~200 µm (0.2 mm). with an optimum temperature of 17 to 23ºC. North of Cape Cod, Massachusetts the clam is predominantly Both the pediveliger and recently-metamorphosed intertidal in distribution, burrowing in the sediments to clams produce elastic byssal threads in a gland within a depth of 30 cm or more. South of Cape Cod the clam the foot. Byssal threads attach the clams to small objects becomes increasingly subtidal and burrows deep in the such as sand grains in the substrate. Young clams can substratum to avoid elevated temperatures. They can be also detach from their byssal threads if necessary and found in waters ≥ 200 m deep. can move along the surface of the sediment pushed by tidal or wind-driven currents. Softshell clams lose their 2 Figure 2. Life cycle of softshell clam. (Drawing by Virge Kask, UConn Biological Illustration.) ability to produce byssal threads as they approach 20 Predation and Disease mm SL. Juvenile clams up to ~20 mm SL are lightweight and do not bury deeply, which means they may be dis- Softshell clam populations are adversely affected by rupted and transported by waves or currents, despite predation, environmental stress and disease. Predators being attached to sand grains by byssal threads. Some- vary with life stage. Larval clams are preyed upon by times, clams appear at the sediment surface to avoid plankton and fish, which if abundant can decimate a predators, anoxic conditions, or for other reasons. If this cohort. Post metamorphic clams are ingested by occurs on sunny days during periods when the tide is nemertean worms, segmented worms, fish, aquatic birds, low, the valve that is exposed to the sun may dry. When crabs, horseshoe crabs, oyster drills, moon snails and the tide returns, the clam will float because of trapped air meiofauna. Surviving clams minimize predation by or increased water tension associated with drying out. growing large and by burrowing deeper into the sedi- The clam may be transported toward the upper intertidal ments. Elevated temperatures during hot summers have area. This action tends to help clams establish them- been associated with mortalities in intertidal populations selves near the upper intertidal zone of some flats. and even subtidally in Chesapeake Bay. Few diseases Softshell clams grow relatively fast. Under optimal have been identified that directly affect softshell clams, conditions they can grow up to 8 mm in a month during unlike oysters and hard clams. Neoplasia, a leukemia- their first summer. Growth is mainly determined by tem- like disease, is relatively common but occurs at very low perature, food supply, current, density and sediment incidence levels (typically less than 5%). At times, how- type. Most growth occurs during the spring, summer ever, neoplasia has been known to cause massive mortal- and fall, with little to no growth in the winter, especially ities. Populations in Prince Edward Island, Canada and in Maine and eastern Canada. Some clams in more in Chesapeake Bay have experienced catastrophic die- southerly waters may become sexually mature by the offs due to neoplasia. Perkinsus spp., the causative agent end of their first summer, and the vast majority of soft of Dermo, has been identified in softshell clams, but its shell clams are ready to spawn the year after they settle impact on clam health remains unclear.